"You say to yourself, 'How much better does it get?'" Michael Balkin, co-manager of the William Blair Small Cap Growth fund, tells The Wall Street Journal.

For some, the answer is not much. More stocks have "eye-popping" valuations than they did six months ago, Aram Green, co-manager of the ClearBridge Small Cap Growth fund, tells The Journal.

JB Taylor, manager of the Wasatch Small Cap Growth fund, is concerned about valuations too. "It wouldn't take much of a break in this overall market to really let the air out of some of the valuations," he explains.

But not everyone is bearish.

"We've got a very low-interest-rate environment, . . . and people are doing anything they can to generate returns, and historically the best place to find returns has been in small caps," Balkin notes.

"Buy a big enough basket of them, and you're bound to end up owning a piece of the next big thing. The trade-off: the interim volatility of small caps is demonstrably higher than that of low-growth, established blue chips, and they get hit harder when the broader market freaks out as it did in 2008."